I think the meaning behind the English slogan "DISCOVER YOUR BODY" is more akin to something like 改めて観察する人体 (Inspect the human body anew).

When I think of the word "発見” I think of stuff like "I discovered(found) that the window was broken" or "I discovered(found) 500 yen in a book" or "10 survivers were discovered(found) in a collapsed building" or "Newton discovered gravity exists"... etc.

In the English slogan, they're using "discover" to mean something more like "to see in a new light" or "to rediscover".

However, since very few people are going to the exhibition to find their own body that is lost or to find anything lost/yet-to-be-found at all, it isn't appropriate to use 発見.

and absolutely! ～しなさい is not appropriate for when a single speaker is talking to several people who are seeing the sign.

Should I be grateful that I can't see the Korean or sad that Korean was left out?

@Herouth, I think you're slightly confused about shinasai. The various ways to request, command, firmly suggest, order, etc, are quite confusing but nasai is the honorific nasaru, you don't use honorific verbs when talking to inferior. shinasai is a firm but polite request.I find Tae Kim's guige to be a very good complement to my books : http://www.guidetojapanese.org/learn/grammar/requestsI still agree that it's probably not the adequate form for this sentence aimed at potential customers. I'd have started to bellify hakken with o-, then added some cute -mase at the end of the conjugation: ohakken sinasaimase. I dunno. For sure I'd have thought about it more than 3 minutes. Skim is right, shinasai isn't good when talking to several people. Maybe ohakken shite kudasaimase?

The Russian is also a bit clumsy too. Unless I'm wrong, it should be откройте, with a short [i] instead of a long one, but more importantly, this verb has 2 meanings : discover, and open. I certainly would have chosen another verb (probably раскрывать) to avoid freaking out people.

@Nicolas: It is very much from superior to inferior. The purpose of the honorific なさる is to soften the imperative (しろ), which is considered to be extremely sharp - basically, them's the fighting words. If you say しなさい, it's still an imperative, but doesn't sound aggressive. But I'd never ever dream of using しなさい to an equal, let alone a superior.

Another example of a honorific softening a command is found in, e.g. 来てらっしゃい ("come!"), which obviously uses いらっしゃる. It is something that a mother would say to her kid. You'd never say that to an equal, with an exception of using it as a stock phrase (like sobaya do, to call people into their shops, where it indeed is honorific).

Finally, the very page you linked says this about 〜なさい: "It is used, for example, when a mother is scolding her child or when a teacher wants a delinquent student to pay attention."